1. Field of the Invention
This invention relates to an electronic imaging apparatus in which the amount of light is adjusted by a system that the internal transmittance of an optical system is changed.
2. Description of Related Art
Recent electronic imaging apparatuses including digital cameras are progressing in compact or slim design. The compact or slim design largely depends on compactness of an electric circuit or of a recording medium. Consequently, the proportion of an optical system to the entire imaging apparatus is relatively increased. Thus, the optical system, notably a zoom lens, is also progressing in compact design through compactness of an image sensor. For example, a collapsible mount type lens barrel is adopted. The collapsible mount type lens barrel is constructed so that the optical system is shifted outward in photography and is incorporated in the body of the electronic imaging apparatus in carrying.
With the progress of the compact design, however, a limit occurs to downsizing of the entire optical system according to compactness of the image sensor. This is because the physical work of lens elements constituting the optical system, the mechanical strength of a system mechanism, and fabrication accuracy are restricted. Consequently, provisions have been made for using aspherical surfaces and high-refractive-index and low-dispersion glass materials to decrease the number of components to a minimum. However, this is also restricted because of the maintenance of the basic specification and correction for aberration. Thus, a reduction in volume or overall length of the optical system or a slim design relating to a depth where the lens barrel is collapsed reaches a limit.
Thus, as approaches of achieving the slim design relating to the depth in particular, the following techniques are available. One of them is to provide a reflecting surface for bending the optical path in an optical system. Another is to use electrochromic elements instead of optical filters and dichroic mirrors. In the latter case, a plurality of electrochromic elements in different wavelength regions of transmission light are combined (refer to, for example, Japanese Patent Kokai No. Hei 11-160739). Such techniques contribute largely to the slim design of the body of the electronic imaging apparatus.
However, in each of these techniques, the problem of diffraction has ceased to be negligible with compactness of the electronic image sensor. For example, when the stop is stopped down above F/5.6, the deterioration of image quality becomes remarkable, thus causing a new problem that the range of adjustment of the amount of light is narrowed. Consequently, techniques of replacing some optical elements in order to lower the transmittance and of using a variable-transmittance optical element have been proposed.